Two-shaft vacuum pump

ABSTRACT

A vacuum pump has two shafts ( 3, 4 ) and two rotors ( 1, 2 ) which co-operate with each other and which are fixed to the shafts. The rotors are cantilevered on the shafts. The rotors are fixed to the shafts in a manner which is devoid of backlash, even during temperature changes. In order to achieve this, the shafts ( 3, 4 ) are made of a material having a modulus of elasticity which is as high as possible, e.g., steel. The rotors ( 1, 2 ) are made of a material having a density which is as low as possible, e.g., aluminum or a titanium alloy. Structures ( 8; 11, 12, 13; 14, 15; 25, 27; 38, 41; 43, 44, 45 ; etc.) are provided to ensure that the rotors ( 1, 2 ) are fixed to the shafts ( 3, 4 ) in a manner which is devoid of backlash at all operating temperatures.

The present invention relates to a vacuum pump comprising two shafts andtwo rotors which co-operate with each other and which are fixed to theshafts. In this manner, the rotors are cantilevered by the shafts.

The developers and manufacturers of prior pumps, screw pumps inparticular, want to operate such pumps at reasonable manufacturing costsat as high as possible speeds, and with leaks through slots as small aspossible, in order to attain the purpose—vacuum generation—aseffectively as possible. The pre-requisites for this are precisebearings and fitting of the rotors to the shafts devoid of backlash—alsoin the warm state. As to the bearing, it needs to be considered that therotors are cantilevered. This is commonly performed two bearings pershaft between which there is located a drive motor. In particular, inthe instance of screw vacuum pumps such a kind of bearing has been foundto be expedient, since its benefits—no seal on the intake side, morecost-effective compared two double-flow solutions—are greater than thedisadvantages—higher requirements as to shaft and bearing.

The cantilevered arrangement is the cause for problems relating toaffixing of the rotors to their shafts devoid of backlash. It is knownthat in the instance of a cantilevered arrangement it is expedient thatthe center of gravity of the rotating system be located in the vicinityof the bearing on the rotor side. This can be achieved in that amaterial being as light in weight as possible, aluminium for example, isselected for the rotor. However, aluminium has a significantly greatercoefficient of thermal expansion (about 23×10⁻⁶/K) compared to steel(12×10⁻⁶/K) which in the case of cantilevered arrangements is speciallywell suited as the material for the shaft. Steel has a high modulus ofelasticity thus enabling the manufacture of stiff shafts. In theinstance of the material pair steel/aluminium it is difficult to affixthe rotor to the shaft devoid of backlash at all operating temperatures(between ambient temperature and approximately 200° C.). There exists,in fact, the possibility of employing as to the expansion problem morefavourable materials like steel, Ti or ceramics for the rotor. However,these result in rotors being too heavy (St) or too expensive (Ti,ceramics). Also aluminium is not a possibility for the shaft materialowing to its low modulus of elasticity.

From DE-199 63 171 A1 a vacuum pump having the aforementionedcharacteristics is known. Affixing of the rotor to the shaft devoid ofbacklash in the warm state is not covered.

It is the task of the present invention to create a vacuum pump havingthe aforementioned characteristics which will optimally fulfil the aimsof the manufacturers and developers of such vacuum pumps.

SUMMARY OF THE INVENTION

This task is solved through the characterizing measures of the patentclaims.

In that the shafts are made of a material having a modulus of elasticitywhich is as high as possible (steel, for example), precise guidance ofthe shafts and thus the rotors is ensured so that the slots between therotors themselves and the housing walls can be kept small. Also themeans which ensure affixing of the rotors to the shafts devoid ofbacklash have this effect. Lighter rotor materials compared to thematerial for the shaft will allow the pump to be operated at highrotational speeds.

The means of ensuring fixing of the rotors to their shafts devoid ofbacklash at all operating temperatures may be implemented differently.In the instance of greater differences between the coefficients ofexpansion of the materials involved, the rotors and the shafts may bedesigned in such a manner that the freedom from backlash is ensuredthrough warm centering, cold centering and/or friction centering. Alsobindings preventing a greater expansion of the aluminium rotor on thesteel shaft are possible. Finally—supported or alone—a coolingarrangement may be present which restricts or prevents temperaturefluctuations at the joints.

As already mentioned, it would be simple to employ materials havingapproximately the same coefficient of expansion. To this end theinventors have proposed to employ aluminium alloys manufactured based onpowder metallurgy, the principal components of which are Cu and Si inthe alloy. Steel and aluminium alloys of this kind have approximatelythe same coefficient of expansion (density of the material—mass) so thatthrough shrink joints of the type commonly employed, fixing of therotors to the shafts devoid of backlash at all operating temperatures isensured.

In order to succeed in placing the center of gravity of the systems eachconsisting of a rotor and a shaft, as close as possible to the bearingon the rotor side for the purpose of attaining high speeds severalmeasures can be expedient:

-   -   Hollow bore in the rotor, into which the steel shaft engages        only partly; if required for the purpose of guiding a coolant        fluid, components having a low density (plastics, for example)        can be accommodated in the bore.    -   Short rotors; this is achieved in screw pumps in a basically        known manner through a suitable change in pitch and/or through        deeply cut-in rotor profiles.    -   Accommodation of the shaft bearing on the rotor side in a recess        on the bearing side within the rotor.    -   O-arrangement of the two shaft bearings and/or movable bearings        at the rotor side, and fixed bearings at the side of the shaft        facing away from the rotor.

Still further advantages will be apparent to those of ordinary skill inthe art upon reading and understanding the following detaileddescription.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating a preferred embodiment and are notto be construed as limiting the invention.

FIG. 1 is a longitudinal sectional view of a single rotor and shaftcombination;

FIG. 2 is a longitudinal transverse view of a pair of shaft and rotorcombinations in an intermeshing relationship mounted to associatedsupport structure;

FIG. 3 is a side view in partial section of an alternate embodiment ofthe rotor shaft combination;

FIG. 4 is a longitudinal sectional view illustrating one embodiment ofbearings for supporting the rotor shaft; and

FIG. 5 is a longitudinal sectional view illustrating an alternatebearing embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the drawing figures the rotors are designated as 1 (resp. 1 and 2 indrawing FIG. 2) and their shafts as 3 (resp. 3, 4). The rotors arecantilevered and equipped with axial hollow bores into which the bareends of the shafts 3, 4 extend. The rotors 1, 2 are each fixed on to theshaft ends devoid of backlash.

In the example of an embodiment in accordance with drawing FIG. 1 therotor 1 has on its face sides two hollow bores 5 and 6 which are linkedto each other approximately at the center of the rotor 1 via a morenarrow bore 7. In the assembled state, the opening of the hollow bore 6on the intake side is firmly sealed with a disk 8, which is—asdepicted—screwed into the hollow bore with the aid of a thread 9, forexample.

In the hollow bore 5 on the bearing side there already ends the shaft 3which is equipped on its face side with an axially oriented collar 11.In the area of the more narrow bore 7 linking the hollow bores 5 and 6,the annular protrusion 12 extending to the inside is equipped with anaxially oriented collar 13, the direction and diameter of which are soselected that it rests from the inside against the collar 11 of theshaft 3. If the shaft 3 is made of steel and the rotor 1 of aluminumhaving, compared to steel, a greater coefficient of expansion and if thecollars 11, 13 rest against each other at ambient temperature devoid ofbacklash, there results an inner centering which remains devoid ofbacklash also at higher temperatures.

For the purpose of joining rotor 1 and shaft 3 there are provided axialbolts 14 which are accessible from the hollow bore 6. These penetratethe protrusion 12 of the rotor 1 and are screwed into the collar 11 ofthe shaft.

Expediently, a ring 15 made of the same material as the shaft isassigned to the heads of the bolts. Thus there results besides warmcentering also friction centering.

Moreover, shaft 3 and rotor 1 are equipped with a system of coolingchannels for the purpose of reducing temperature related problems. Tothis end the shaft 3 is equipped with a central bore 16. Located in thisbore 16 is a pipe section 17 which extends into the hollow bore 6 andwhich serves the purpose of feeding in a coolant. Within the hollow bore6, hollow (thin walled) and/or light installations 18 affixed to pipesection 17 form an outer annular channel 19, which among other things,is linked via the bore 7 to an outer annular channel 21 in the hollowbore 5 formed by the shaft 3 and the inner wall of the hollow bore 5.Via these annular channels 19, 21 and thereafter via the annular channel23 in the shaft being provided by pipe section 17 and the inner wall ofthe bore 16, the coolant flows back. A reverse direction for the coolantflow may also make sense.

In drawing FIG. 2 the rotors 1, 2 are equipped on the bearing side withcollars 25, 26, said collars encompassing the shafts 3, 4 from theoutside. If the rotor material has a greater coefficient of expansionthan the shafts, backlashes may be present between rotors and shaft whenthe temperatures increase in the instance of outer centering of thiskind. In order to avoid this, rings 27, 28 are provided which in turnencompass the collars 25, 26. If the coefficient of expansion of thematerials for the rings 27, 28 is equal or even smaller than thecoefficient of expansion of the material for the shaft, rings 27, 28will at increasing temperatures prevent an expansion of the collars 25,26 and thus the undesirable backlashes.

A cooling system in accordance with the cooling system of drawing FIG. 1is provided. The annular channels 21, 22 extend up into the areas of thecollars 25, 26. Said annular channels reduce the maximum operatingtemperatures which may occur and thus equally remove the risk ofbacklashes.

From the outside the rings 27, 28 are equipped with annular grooves inwhich piston rings which are not depicted, are located. These formjointly with the rings 29, 30 affixed to the housing, labyrinth seals31, 32 which serve the purpose of preventing the ingress of lubricantvapours from the bearings 33, 34 into the pump chambers 35, 36 of thescrew pump.

In the example of an embodiment in accordance with drawing FIG. 3,frictional centering has been implemented. To this end a disk 38 isprovided which initially has the task of sealing off the opening of thehollow bore 5 on the intake side. The disk 38 is firmly joined to boththe shaft 3 (bolt 39) and also the rotor (several bolts 41). If therotor material has a greater coefficient of expansion compared to shaft3 and if the disk 38 consists, for example, of the shaft material, thenthe fixed bolted joint will prevent the formation of backlash atincreasing temperatures.

As depicted in drawing FIG. 3 the disk 38 may be equipped with anaxially oriented collar 43 which engages into the hollow bore 5. Thus atthe same time warm centering can be attained. To this end, it isrequired that rotor 1, shaft 3 and disk 38 be fitted without backlash inthe warm state. Due to the already mentioned conditions with respect tothe coefficients of expansion, this type of mounting is devoid ofbacklash at decreasing temperatures. This also applies to fixing of therotor/shaft without disk 38.

Fixing of the rotor to the shaft may also be effected by means of apress fit joint. If the rotor consists of aluminium and the shaft ofsteel, then it is in this instance expedient that the ambienttemperature at which this press fit joint is manufactured, correspondsapproximately to the maximum temperature encountered by the rotors (1,2) which occurs during operation of the two-shaft vacuum pump.

A joint of this kind is devoid of backlash at all occurring operatingtemperatures of the two-shaft vacuum pump.

Also depicted in drawing FIG. 3 is that the collar 43 and the face sideof the shaft 3 rest against each other, preferably within an outerrecess 44 in the shaft 3. Located between the facing supporting surfacesof collar 43 and shaft 3 is an adjusting ring 45. By inserting adjustingrings 45 differing in thickness—or through collars 43 differing inheight—the axial position of the rotor 1 with respect to shaft 3 can bedefined. Thus there exists the possibility of adjusting flank-to-flankbacklash of the rotor 1 with respect to the second rotor not depicted.Disk 38 may simultaneously serve the purpose of balancing and/or torquetransfer (by way of a tooth lock washer, for example).

Finally depicted in drawing FIG. 3 is the possibility of arranging thebearing 33 on the rotor side in a recess 47 at the bearing side in rotor3. An axially extending bearing support 48 engages into the recess 47.The system of cooling channels (bore 16 in the shaft 3, pipe section 17)extends up to bearing 33 so as to maintain the bearing temperatures at alow level.

In order to reliably attain the desired high speeds it is expedient thatthe two shaft bearings 33, 51 have an O type arrangement as depicted indrawing FIG. 4. In bearings of this kind the point of application of theforce is shifted by the pressure angle in the direction of the rotor'scenter of gravity. In view of this, also a movable bearing 33 at therotor side and a fixed bearing 51 at the side of shaft 3 facing awayfrom the rotor is expedient. Drawing FIG. 5 depicts this arrangement.The point of application of the force is at the bearing center.

The invention has been described with reference to the preferredembodiment. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

1. A vacuum pump including: two steel shafts; two aluminum rotors, eachhaving a hollow bore; a tight friction joint formed between each of theshafts and a corresponding rotor by press fitting the shaft into thecorresponding rotor bore with the rotor heated at least to a maximumoperating temperature attainable during vacuum pumping such that thealuminum rotors are fixed to the steel shafts in a manner which isdevoid of backlash at all operating temperatures; a cooling meansdisposed at a level at which the shaft and the rotor are joined; and theshafts being rotatably mounted with the rotors cantilevered on theshafts and co-operating with each other.
 2. The pump according to claim1, wherein the rotors are made of aluminum alloy manufactured based onpowder metallurgy, components of which alloy include Cu and Si.
 3. Avacuum pump comprising: two steel shafts; two aluminum rotors whichco-operate with each other and which are fixed to the shafts, the rotorsbeing cantilevered on the shafts; a cooling means disposed at a level atwhich the rotors and shafts are joined; and, a means for insuring thatthe rotors are fixed to the shafts in a manner which is devoid ofbacklash at all operating temperatures, including a tight frictionaljoint formed by: drilling a hollow bore in each of the aluminum rotors;heating each of the rotors to a maximum operating temperature which therotors attain during vacuum pumping; inserting a corresponding one ofthe steel shafts into the hollow bore of each rotor; and press fittingeach of the rotors heated to the maximum operating temperature and thecorresponding shaft into the tight frictional joint.
 4. The pumpaccording to claim 3, wherein the rotor is equipped with a collar whichencompasses the shaft and further including: a binding which encompassesthe collar.
 5. The pump according to claim 3, further including: a rotorside bearing located in a recess in the rotor.
 6. The pump according toclaim 3, further including: two bearings mounted on each shaft with an Otype arrangement.
 7. The pump according to claim 3 further including: amovable bearing adjacent to the rotor; and a fixed bearing remote fromthe rotor.
 8. The pump according to claim 3, further including: a diskarranged on an intake side of the rotor.
 9. The pump according to claim8, wherein the disk is equipped with a collar engaging into the hollowbore of the rotor, said disk effecting cold centering.
 10. The pumpaccording to claim 3, wherein the cooling means includes a hollowinterior space defined in the rotor bore and the shaft only partlypenetrating the hollow space.
 11. The pump according to claim 10 furtherincluding: light-weight components disposed in the hollow space betweenthe shaft and the rotor to guide a coolant flow.
 12. A vacuum pumpcomprising: two shafts; two rotors which co-operate with each other andwhich are fixed to the shafts, the rotors being cantilevered on theshafts, having hollow bores, and being of a material having a lowerdensity than the shafts; and, a disk arranged on an intake side of eachrotors, each disk being equipped with a collar engaging into a hollowbore of a corresponding one of the rotors, said disk effecting coldcentering; the collar and the shaft resting against each other directlyand through an adjusting ring; a means for insuring that the rotors arefixed to the shafts in a manner which is devoid of backlash at alloperating temperatures.
 13. The pump according to claim 12, furtherincluding: a means for at least one of cold centering, warm centeringand friction centering the rotor on its shaft.
 14. The pump according toclaim 13, wherein the means for warm centering includes: axiallyextending collar sections of the rotor relative to the shaft, the collarsection of the rotor being located inside the rotor.
 15. The pumpaccording to claim 13, wherein the means for friction centeringincludes: axially oriented bolts which join the rotor and the shaft toeach other.
 16. A vacuum pump comprising: a pair of steel shafts; aplurality of bearings which support the shafts in a parallelrelationship, the bearings being mounted adjacent a first end of theshafts such that second ends of the shafts are cantilevered; a pair ofaluminum rotors which have a different coefficient of thermal expansionthan the shafts, each of the rotors having an internal bore, a secondend of each of the shafts being received in one of the bores to supportthe rotors in an intermeshing relationship with each other; a means forinhibiting backlash at elevated operating temperatures including: eachshaft press fit into the bore of one of the rotors when the rotor washeated to at least a maximum operating temperature to form a tightfrictional engagement at temperatures at and below the maximum operatingtemperature, a cooling passage extending through each shaft to a regionin which the shaft and rotor are in tight frictional engagement to limittemperature in the region of tight frictional engagement during vacuumpumping.
 17. A vacuum pump comprising: a pair of shafts; a plurality ofbearings which support the shafts in a parallel relationship, thebearings being mounted adjacent a first end of the shafts such thatsecond ends of the shafts are cantilevered; a pair of rotors of a lowerdensity material than the shafts and which has a different coefficientof thermal expansion than the shafts, each of the rotors having aninternal bore, a second end of each of the shafts being received in oneof the bores to support the rotors in an intermeshing relationship witheach other; a means for inhibiting backlash at elevated operatingtemperatures including at least one of: a flange defined on an interiorsurface of each rotor extending into the bore parallel to the shaftswith an axially extending collar section of each shaft press fit betweenthe flange and a wall surface of the bore in a tight frictionalengagement at the operating temperature, an inward projecting collarsection of the rotor and axially oriented bolts extending through thecollar section and threadedly received in the shaft, a disk frictionallyengaged in each bore and attached to the rotor which is received in thebore, a disk mounted on an intake side of each rotor and anchored to theshaft and to the rotor, a disk with a collar which collar is press fitat the elevated operating temperature into engagement with the bore ofthe rotor with the collar engaging the shaft, a collar defined on oneend of the rotor and a peripheral binding which surrounds the rotorcollar, the binding having a coefficient of thermal expansion that iscomparable with the coefficient of thermal expansion of the shaft, atleast one region of firm frictional engagement between an outer surfaceof each shaft and an inner surface of each rotor bore with a coolingduct extending through the shaft therepast to cool the frictionallyengaging portions of the shaft and rotor, at least one ring of materialwith the same coefficient of thermal expansion as the shaft mounted ineach rotor bore, a disk with the same coefficient of thermal expansionas the shaft anchored to each rotor to limit outward radial thermalexpansion of each rotor.
 18. The vacuum pump according to claim 17wherein the shaft is made of steel and the rotor is made of a lightweight alloy including at least one of aluminum, titanium, and ceramics.